i have a 86 cherokee and i put a new brake boster ont and win u put on tha brake it sounds like air is leakin and tha brake's grip hard

 

Is it an OEM booster? Check your vacuum lines as well. An easy way to check for any pinhole leaks on the booster is to spray soapy water on it while your engine is on and check for bubbles. Be sure not to put your fingers any where near the booster while its energized. If it has a leak it could pierce your skin and put bubbles in your blood.